Designs, Codes and Cryptography ( IF 1.4 ) Pub Date : 2024-12-05 , DOI: 10.1007/s10623-024-01535-2 Xin Wei, Xiande Zhang, Gennian Ge
Set systems with strongly restricted intersections, called \(\alpha \)-intersecting families for a vector \(\alpha \), were introduced recently as a generalization of several well-studied intersecting families including the classical oddtown and eventown. Given a binary vector \(\alpha =(a_1, \ldots , a_k)\), a collection \({\mathcal {F}}\) of subsets over an n element set is an \(\alpha \)-intersecting family modulo 2 if for each \(i=1,2,\ldots ,k\), all i-wise intersections of distinct members in \({\mathcal {F}}\) have sizes with the same parity as \(a_i\). Let \(f_\alpha (n)\) denote the maximum size of such a family. In this paper, we study the asymptotic behavior of \(f_\alpha (n)\) when n goes to infinity. We show that if t is the maximum integer such that \(a_t=1\) and \(2t\le k\), then \(f_\alpha (n)\sim (t! n)^{1/t}\). More importantly, we show that for any constant \(c>0\), as the length k goes larger, \(f_\alpha (n)\) is upper bounded by \(O (n^c)\) for almost all \(\alpha \). Equivalently, no matter what k is, there are only finitely many \(\alpha \) satisfying \(f_\alpha (n)=\Omega (n^c)\). This answers an open problem raised by Johnston and O’Neill in 2023. All of our results can be generalized to modulo p setting for any prime p smoothly.
中文翻译:
在具有严格限制交叉点的现场系统
具有严格限制交集的集合系统,称为向量 \(\alpha \) 的 \(\alpha \) 相交族,是最近引入的,作为几个经过充分研究的相交族的推广,包括经典的 oddtown 和 eventown。给定一个二进制向量 \(\alpha =(a_1, \ldots , a_k)\),如果对于每个 \(i=1,2,\ldots ,k\) ,\({\mathcal {F}}\) 中不同成员的所有 i 向交集的大小与 \(a_i\) 相同,则 n 个元素集上的子集 \({\mathcal {F}}\) 是一个 \(\alpha \) 相交的族模 2。设 \(f_\alpha (n)\) 表示这样一个族的最大大小。在本文中,我们研究了当 n 趋于无穷大时 \(f_\alpha (n)\) 的渐近行为。我们表明,如果 t 是最大整数,使得 \(a_t=1\) 和 \(2t\le k\),那么 \(f_\alpha (n)\sim (t! n)^{1/t}\)。更重要的是,我们表明,对于任何常数 \(c>0\),随着长度 k 变大,几乎所有 \(\alpha \) 的 \(f_\alpha (n)\) 都以 \(O (n^c)\) 为上限。等价地,无论 k 是什么,满足 \(f_\alpha (n)=\Omega (n^c)\) 的 \(\alpha \) 数量有限。这回答了 Johnston 和 O'Neill 在 2023 年提出的一个悬而未决的问题。我们所有的结果都可以顺利地推广到任何素数 p 的模 p 设置。